1. Field of the Invention
The present invention relates generally to membrane switches, and particularly to a membrane switch in which migration of metallic ions among contact points due to moisture is suppressed.
2. Related Art
A membrane switch 200 having a structure shown in FIGS. 8A and 8B is well known. Such a membrane switch 200 includes two opposing flexible printed circuits (hereinafter referred to as FPCs) 21, 22 separated by a predetermined distance. When pressure is applied to a contact part (region indicated by X in FIG. 8B), the FPCs 21, 22 contact each other, and conduction occurs.
FPCs 21, 22 are composed, for example, of resin films 211, 212, such as polyethyleneterephthalate (PET), having printed or laminated thereon highly conductive metallic conductive layers formed from copper or silver, such as those shown at 221, 222. After the metallic conductive layers are laminated to the resin films, an electrical circuit is formed thereon by, for example, etching.
The resulting circuit forms a contact part indicated by region X, an inner wiring part indicated by region Y, and an outer wiring part indicated by region Z which connects the inner wiring part Y to an outer circuit (not shown).
While a thick copper or silver film exhibits excellent conductivity, the resistance of such a film increases as oxidation and corrosion of the metallic material occurs. Therefore, resin films 231, 232, which are conductive due to dispersion of carbon particles therein, are formed as protective layers on the metallic conductive layers 221, 222. The resin conductive layers 231, 232 cover the metallic conductive layers 221, 222, respectively, to protect the metallic conductive layers from oxidation and corrosion. Thus, the metallic conductive layer 221 and the resin conductive layer 231, as does the metallic conductive layer 222 and the resin conductive layer 232, form a conductive part of the switch.
When pressure is applied to the X region, the resin conductive layers 231, 232 contact each other, but the metallic conductive layers 221, 222 do not contact each other. Hereinafter, the metallic conductive layer and a non-metallic conductive layer, such as the resin conductive layer, will together be referred to as a conductive part.
Further, in the membrane switch 200, the FPCs 21, 22 sandwich a spacer 24. The spacer 24 is typically formed from an insulating material having a prescribed thickness so that the opposing contact parts X of the FPCs 21, 22 are separated by a predetermined distance. Therefore, after lamination, a cavity 240 between the sealed contact parts is formed by the contact parts X and a spacer side wall 241.
When pressure is applied to the contact parts X, the resin films 211, 212 are deformed so that contacts 261, 262 on the surface of the resin conductive layers 231, 232 contact each other to form an ON state. When the pressure is removed, the contacts 261, 262 are separated from each other to form an OFF state.
However, because the two FPCs 21, 22 are laminated via an adhesive, a minute gap is often formed between two or more of the FPC layers during lamination. Therefore, when the membrane switch 200 gets wet, water may reach the cavity 240 through these minute gaps. Similarly, under high humidity conditions, water vapor may penetrate the membrane switch through a breathe hole (not shown) provided to facilitate stable mechanical operation of the contacts, resulting in water condensation in the cavity 240. Furthermore, water may become trapped inside the membrane switch as the switch is washed during the manufacturing process, and as a result dew condensation may occur in the cavity during low temperature conditions.
When water is present in the cavity 240 and on the side wall 241, it is repeatedly subjected to vaporization and condensation, and gradually penetrates the resin conductive layers 231, 232. As a result, some of the metal contained in the resin conductive layers 231, 232 is ionized.
When an electric field is applied to the contact parts X for a long period of time under such conditions, metallic ions can be transmitted from the metallic conductive layer of the positive electrode 221 (or 222) through the resin conductive layer 231 (or 232). The transmitted metallic ions form metallic crystals on the side wall 241, which gradually grow from the metallic layer of the positive electrode to the metallic layer of the negative electrode due to a leakage current. As a result, a so-called migration of these metallic crystals occurs. Eventually, the migration causes the pair of electrodes to come in contact with each other, and a short-circuit current I flows across the electrodes, causing apparatus malfunction.
To prevent the above-discussed migration, the metallic conductive layers 221, 222 may be formed only on the outer wiring part Z, with the metallic conductive layers 221, 222 not being formed on either the contact part X or the inner wiring part Y. However, because the amount of carbon particles that can be dispersed in a resin has an upper limit, it is impossible to sufficiently increase the conductivity of the resin conductive layers 231, 232 if so utilized. Furthermore, adherence of the resin conductive layer to the resin film is generally inferior to that of the metallic conductive layer. Therefore, a membrane switch that does not have a metallic conductive layer in the FPCs 21, 22 on the contact part X and the inner wiring part Y cannot be practically used.